When a single drop of fuel can decide the winner of a vehicle economy competition, excellent aerodynamics is critical. That is what we realized at the Department of Mechanical Engineering of the University of Coimbra when we decided to enter the annual Shell Éco-Marathon contest in 1998.

In this event, each prototype must cover six laps of the Nogaro circuit, at a minimum average speed of 25 km/h with only gasoline, diesel or LPG as permitted fuels. The volume of fuel spent is then measured and an extrapolation is made for the distance the car would travel with one liter of fuel. With the overall drag of a vehicle being determined by the small difference between positive and negative forces, numerical prediction is very sensitive to the accuracy of the numerics. Flow separation can compound these difficulties, though in our case, the car shape was designed to avoid any separation.